1. Field of the Invention
This invention relates to a process of three dimensional lithography in amorphous polymers to form an instantaneous, permanent image in the polymer. More particularly, the invention relates to a process of imprinting permanent optical data readable images, through a mask, with infrared energy.
2. Description of the Prior Art
The development of media for the optical recording of digital and image information has been a topic of active investigation for the past several years. In the storage of digital information the desired result is achieved by selectively creating pits, cracks, or bubbles in an organic or inorganic medium. One area for the commerical application of optical data storage is making video disks for the reproduction of movies and compact disks (CD) for the reproduction of music and voice. Of course, today, CD's are also used for the storage of images such as book pages, drawings, photographs and the like.
Industrial production of compact discs involves creating a metal master die and stamping out polycarbonate disks with the master to create the desired array of pits. The stamped discs are then metalized and coated with a protective layer. Such systems are illustrated in U.S. Pat. No. 4,141,731 issued to Jarsen on 27 Feb. 1979 and U.S. Pat. No. 4,615,969 issued to Strand on 7 Oct. 1986.
The master die has a limited lifetime. Repeated pressings dull the die and affect the quality of sound reproduction. An alternative is needed which will permit the rapid, reproducible "printing" of optical data, particularly CD disks without the attendant deterioration of the "printing" instrument.
An alternative to creating the "marks" in the polymer with a metal die is to "print" the image by some form of a lithographic process. In such a process, images are usually produced by taking advantage of the photosensitivity of the substrate or, more commonly, on a compound coated on the substrate. The material will acquire the "written" image by several mechanisms including solubilization or insolublization of exposed areas and ablation of exposed areas. The compounds utilized for these purposes are patented mixtures of polymers and additives.
In a related application by the same inventors as this application a process is described of reversibly writing on an amorphous polymer surface. The related application is titled, "Immediate Write, Read, and Erase Optical Storage Medium and Method of Marking", U.S. patent application Ser. No. 07/422,723 filed 16 Oct. 1989, now U.S. Pat. No. 4,975,358. The present invention is an extension of the methods described in the previous application to the permanent marking of the polymer film.
The prior art describes many attempts to develop an optical writing method. Pearson, in an article titled "Polymeric Optical Disk Recording Media", CRC Critical Reviews in Solid State and Material Science, Vol. 13, pp. 1-26 at page 1 (1986) stated
High-Speed, instantaneous optical recordings by use of a laser is finally emerging from the laboratory as a powerful new technology for information storage. The technique offers the capability for low-cost-high-capacity data storage with rapid, random-access retrieval. PA1 1) High-marking sensitivity at an appropriate wavelength coupled with long-term stability of the recording medium and the recorded information; PA1 2) High signal-to-noise ratio (SNR) of the recorded information to facilitate data retrieval; PA1 3) Low defect density level to minimize errors; PA1 4) Substantially instantaneous readability (microseconds) following the marking event (direct read after write [Draw]).
As the demand increases for improved techniques for both permanently writing data and for writing/erasing data, the search continues for materials which can alternately provide permanent recording (ROM) capabilities and materials which can provide write and erase capabilities. Polymeric optical disks are emerging as one of the leading candidates for such a recording medium.
The Pearson article infra excellently summarizes the state-of-the-art in the field and presents the criteria for an acceptable medium including:
These criteria eliminate materials requiring post image processing or slow image development.
A vast number of polymers and modified polymers to meet these criteria have been suggested, but a fully satisfactory solution has not been found.
An optical reading system usually comprises a means of projecting light at the recorded surface and a means of detecting the reflections from the surface. The data stored for optical reading must be marked onto the storage medium in such a way that the reading light projected at the surface can be varied or broken up to cause changes in signal corresponding to the nature of the data.
Basically, there are four types of changes in the surface character used with polymeric optical disks. The first is to form a shallow pit. The second is to form a deep pit. The third is to form a blister and the fourth is to cause changes the optical but not the chemical characteristics of the surface.
The first three methods usually permanently deform the surface. In two of these ("ablative techniques"), material is either displaced or moved to the side by localized heating of the material. In the case of the deep pit, material is literally blasted out or removed from the localized area. The ablative techniques are most useful for permanent markings.
In the blistering technique, a bubble or bump is formed in the material itself or at the interface of a sandwich or composite of material. These techniques are usually Read-Only (ROM) techniques.
The fourth technique changes the optical characteristics of a localized area. This may be done by having dyes embedded in the polymers which dyes will convert between an amorphous and a crystalline state causing a change in the color or clarity of the polymer or by changing the materials themselves. Pearson, infra at page 16, describes the use of dyes in write/erase systems.
It has been suggested that dyes can be used in polystyrene oligomer to form a write/erase medium. This system is an ablative system in which a pitted surface can be regenerated to a smooth surface by heating the polymer. Of course, a little material is lost in each cycle so the number of repetitions is limited to the point where a hole is driven through the recording medium. These techniques are described by Pearson, infra, at pages 16 and 17.
Phase change has been shown in the literature for several of the different techniques. In some cases, a polymer is heated rapidly to vaporize some component of the polymer film, forming a bubble. These bubbles may or may not be partially crystalline; the crucial feature is that the bubble material be rigid in order to stabilize the bubble. Materials used in this approach are polymethylmethacrylate (a non-crystallizing polymer), polymethylstyrene, and polycarbonate. Illustrative of these techniques are U.S. Pat. No. 4,360,895 (1982) to Cornet and European patent No. 58,496 to Maffit, Robbins, and Wilson (1982). An optical data storage system described by Willis in U.S. Pat. No. 4,264,986 employs a similar concept but adds the erase feature. Images can be erased by heating the rigid bubble to a high temperature, whereby the resilience of the material allows it to contract, followed by rapid quenching of the polymer back to a rigid state. A similar approach is described by Lind et al. in U.S. Pat. Nos. 4,780,867 and 4,719,615.
Another phase change recording layer is disclosed in Japanese Patent No. 58-199,345 to Ota, K. (1984). The active medium consists of a transition metal diketonate dispersed in a polymer matrix. Within the polymer, light sensitive domains comprised of the metal diketonate can be converted from the amorphous to the crystalline state with a pulsed N.sub.2 laser. The image formed can be erased by heating.
Small molecule (i.e., non-polymeric) inorganic materials such as oxides of tellurium (Takenaga et al., J. Appl. Phys., 54, p. 5376, (1983)), alloys of GeTe, SeTe, and ternary alloys such as varying ratios of TeGeSb (Ovshinsky and Fritzsche, IEEE Transactions on Electron Devices, 20, p. 91, (1973)) are capable of being switched from the amorphous to crystalline state by application of a short pulse of electricity or light. These non polymeric materials can be repeatedly cycled. This work is further developed in 133 patents issued to S. R. Ovshinsky. These are exemplified by U.S. Pat. Nos. 4,226,898, and 4,217,374.
There are many patents and articles which describe ablative techniques. Novotny and Alexandru, J. Appl. Polymer Science, Vol 24, pp. 1321-1328 (1979), describe a dye-polymer system based on the thermal diffusion of the dye into a PET (Mylar) film substrate following exposure to 1 microsecond of 32 milliwatt 5145 .ANG. (angstrom) radiation. The small dye spots, whose spatial profile is similar to that of the exciting beam, can then be read by a variety of means. Murthy, Klingensmith, and Michl, J. Appl. Poly. Sci., 31, p. 2331, (1986), describe a method of optical data storage that involves exposure of deformed (stretched) poly(vinyl chloride)-dye films to laser radiation at 823 nm. The heating caused a loss of film birefringence which can be detected by a variety of means.
Lind et al. in U.S. Pat. No. 4,780,867 describes an optical recording mechanism whereby an initially amorphous polymer (an expansion layer) is adhered to a substrate (a retention layer). Bubble formation creates a localized deformation. This deformation can then be erased by reheating the polymer expansion layer and using the shear deformation induced by its attachment to the retention layer to "pull" the surface smooth. A similar disclosure appears in U.S. Pat. No. 4,719,615 by the same assignee.
The marking of polymer films with three dimensional resolution is described in the patent literature of photoresist technology. Recent reviews of this field include: C. G. Willson and M. J. Bowden in Electronic and Photonic Applications of Polymers, Adv. Chem. Ser., American Chemical Society, Vol. 218, 1988, Chapter 2; Symposium on Polymers in Information Storage Technology, Polymer Preprints, Vol. 29, p. 195, 1988; Symposium on Polymers in Microlithography, Polymeric Materials: Science and Engineering, Vol. 60, p. 40 1989. These processes differ from optical data recording because:
they have a chemical development step in the production of an image;
the materials used are not commercially available and inexpensive;
they are constrained to a particular wavelength;
the theoretical resolution is less than 1000 .ANG. and the demonstrated resolution is less than 10,000 .ANG. (1 .mu.);
The lithography art has utilized differences in reactivity on a polymer surface created by exposure of a polymer to heat to "process" an image. Rubner, in U.S. Pat. No. 4,486,463, describes a process for the selective metal plating of RYTON.RTM., a clay filled poly[phenylene sulfide] sold by Phillips. The RYTON.RTM. polymer surface is exposed through a mask to heat generated by a CO.sub.2 laser. An image is generated which is sequentially made hydrophilic, catalyzed, doped to deactivate catalyst in certain regions and selectively metal plated by electroless plating. Rubner utilizes the change in state of RYTON.RTM. from amorphous to crystalline to create regions in which the dopant is excluded in order to form the image. RYTON.RTM. does not provide sharp images because the laser generated heat is rapidly and efficiently dissipated to the surrounding polymer.
Image storage via ablation of polyethylene is described by Kuder et al., J. Appl. Polym. Sci, 35, p. 1257 (1988). Kuder exposes prepared film to intense radiation from a HeNe laser resulting in 10 micron holes. Bell describes forming high resolution pits in inorganic layers of Ti/MgF.sub.2 /Al, Bell, A. E. J. Appl. Phys, 53, p. 3438 (1982). Howe and Wrobel., J. Vac. Sci. Technol., 18, p. 92, (1981), describes the use of laser induced ablation in dye-polymer-Al structures.
There is a preference in the optical data storage field to use shortwave lengths in the UV or far UV range because of the belief that these wavelengths are necessary to obtain sharp images. The belief is that the size of the image can't be less than the wave length of the radiation used to imprint. The reasons for this preference are explained by Iwayanagi et al., "Materials and Processes for Deep-UV Lithography", Electronic and Photonic Applications of Polymers, Adv. Chem. Ser., American Chemical Society, Vol. 218, 1988, Chapter 3, at pages 109-110. Srinivasan et al., Appl. Phys., 41, No. 6, pp. 576-578, 15 Sept. 1982, exemplifies the use of UV to form an image. In addition, in U.S. Pat. No. 4,417,948, Srinivasan and Mayne-Banton describe a deep ultra-violet wavelength technique for photomarking polyester films (including PET) which is reported to work in reactive or inert atmospheres. These are ablative techniques based on photoetching techniques.
Andrew et al., in an article titled "Direct Etching of Polymeric Materials Using a XeCl Laser", Appl. Phys. Lett., 43(8), pp. 717-719, 15 Oct. 1983, suggests that the UV etching in PET "is primarily due to a thermal process." (at page 717). As proof, Andrew et al. states "It is relevant to note that consistent with the proposed thermal model a similar structure to that in FIG. 2 was revealed by "etching" using a 30-ns TEA CO.sub.2 laser turned to a strong absorption band in PET. The excimer laser is thus not unique in its ability to etch and reveal microstructure but has the advantages of short penetration depth, short pulse length, and a low threshold fluence which allows very controllable material removal. "Andrew et al. at page 719.
E. O. Chiello, F. Garbassi, and V. Malatesta, Appl. Macromolecular Chem. and Physics, 169, p. 143, (1989), describe using a CO.sub.2 laser to irradiate what are initially opaque PEEK films (0.25 mm thick). The irradiation causes hole formation (&gt;30s at 100 W) or surface disruption by deformation and melting (&lt;20s at 10 W). In the latter case, the deformation resulted in "the only relevant feature is some corrugation of the surface, probably due to microfusion events". The authors noted that the low power irradiation caused melting and that upon turning off the laser "the polymer remained deformed, but the treated zone required (sic) the original opaque appearance." Recently, Fang et al., Polymer Engineering and Science, Vol. 29, No. 18, pp. 1241-1245 (September 1989) at p. 1245, noted the phenomena which is the basis of this application. Fang does not teach how to effect the expressed desired result.